Polymer coating for low electrically conductive materials

ABSTRACT

A process for forming a coating of a thermoplastic polymer on a hollow object formed of a low electrically conductive material is described, characterized in that the thermoplastic polymer has its maximum IR absorption between 1300 and 1900 nm and/or 2800 and 3000 nm, is applied electrostatically as a powder to the object while the objet is held at a temperature below the melting point of the thermoplastic polymer, and thereafter the object is heated by IR radiation to form the coating.

The present invention relates to a process for providing a hollow objectformed of a low electrically conductive material with a coating of athermoplastic polymer, and in a preferred embodiment, providing a glassor ceramic container with a coating of an ionomeric polymer.

BACKGROUND OF THE INVENTION

Polymer coatings are provided on objects having low electricalconductivity such as ceramics and glass for a variety of reasons such asfor weight savings, shatter protection, aesthetics and touch. Inparticular, providing shatter protection to glass bottles containingcarbonated beverages is of particular importance where the glass bottlesare defective or mishandled.

Polymeric coatings can be applied by various means. The objects to becoated can be dipped into a polymeric melt or can be dipped into afluidised bed of polymer powder particles. It is also known to coatobjects using an electrostatic powder spraying process; c.f. DE 2424220,DE 2750370 C2 and DE 2750372 C2.

DE 2750370 C2 and DE 2750372 C2 disclose electrostatic spray coatingprocesses in which the substrate (a glass bottle) is maintained at atemperature below the melting point of the polymer, and the powder isapplied by way of silent discharge. Afterwards, the bottle is heated tomelt the powder to form a coating, which is subsequently cured. Theseprocesses, however, require high investment costs, can lead to corrosionof the equipment as they require continuous discharge, have limitationsto the speed at which they can be carried out, require the use of veryfine polymer powder which is expensive to produce, often producenon-uniform coatings due to local discharges at the bottle surface, andcan be dangerous to carry out due to shattering of the bottles duringheating and quenching steps.

These disadvantages are overcome by the current invention.

DETAILED DESCRIPTION OF THE CURRENT INVENTION

The present invention provides a process for forming a coating of athermoplastic polymer to a hollow object formed of a low electricallyconductive material, characterised in that the thermoplastic polymer hasits maximum IR absorption between 1300 and 1900 nm and/or 2800 and 3000nm, is applied electrostatically as a powder to the object while theobject is held at a temperature below the melting point of thethermoplastic polymer, and thereafter the object is heated by IRradiation to form the coating.

Advantages of the process include

the hollow objects, e.g. glass bottles, can be taken directly from theannealing ovens of the glass bottle plants, thereby avoiding the needfor large, expensive to preheat the objects prior to coating;

coating output is increased considerably;

potentially dangerous electric discharge and vacuum equipment is notneeded;

more uniform coatings can be obtained using less expensive, largerparticle size polymer.

As mentioned, suitable thermoplastic polymers to be used in the processaccording to the invention will have their maximum IR absorption between1300 and 1900 nm and/or 2800 and 3000 nm. Preferred such polymers areethylene/acid copolymers and ethylene/acid/acrylate terpolymers andtheir corresponding ionomers.

Ethylene/acid copolymers and their corresponding ionomers are well knownin the art to be copolymers of ethylene with an olefinically unsaturatedorganic acid such as acrylic or methacrylic acid, the acid comprisingabout 1 to 50 mole percent of the total polymeric material. Theethylene/acid copolymers and their methods of preparation are well knownin the art and are disclosed in, for example U.S. Pat. Nos. 3,264,272,3,404,134, 3,355,319 and 4,321,337. The copolymers are termed ionomerswhen the acid is neutralized in whole or in part, preferably up to 80%to produce a salt. The cation for said salts may be a metal ion chosenfrom the first, second or third group of the periodic table of elements,and is preferably an alkali metal such as sodium, potassium or zinc.

Ethylene/acid/acrylate terpolymers and corresponding ionomers are wellknown in the art to be terpolymers of the aforementioned ethylene andolefinically unsaturated organic acids, together with an alkyl acrylateor methacrylate (e.g. ethylacrylate, n-butyl acrylate, isobutylacrylateor methacrylate). The terpolymers will typically contain between 1 and40 weight % of alkyl acrylate or methacrylate, 1 and 50 weight %unsaturated organic acid and 50 and 98 weight % ethylene.

Preferred ionomers are copolymers of ethylene and 1 to 50%, morepreferably 5 to 25%, by weight acrylic or methacrylic acid neutralizedup to about 90%, more preferably from 5 to 60% with an alkali metal ionor a divalent or trivalent metal ion, the melt index of the copolymer,whether neutralized or unneutralized, being about 0.1 to 60, preferably20 to 40, dg/min. according to ASTM Standard D1238 (condition E).

Suitable acid copolymers and ionomers are available from the DuPontCompany under the trade names Nucrel® and Surlyn®, respectively, and aregenerally described U.S. Pat. Nos. 3,264,272 and 4,351,931.

The thermoplastic polymer powders used in the present invention willtypically have a particle size up to about 300 microns, and willpreferably be from 100 to 250 microns in size. These polymer powders canbe made by usual techniques, such as cryogenic grinding.

Equipment for carrying out electrostatic spray coating and its generaloperation is known.

A general procedure for the process according to the present inventionis as follows. A hollow object, e.g. glass bottle, maintained at atemperature below 100° C., is placed into an electrostatic spray boothwithout preheating. In the spray booth, a steel rod is placed in contactwith the inside of the bottle, thereby establishing an electrostaticpotential between powder to be applied to the bottle and bottle. Thepowder is electrically charges and applied to the bottle in a uniformmanner. The bottle is then placed either in an oven (conventional or IR,according to the example followed) to melt the polymer powder. The IRoven will be configured to permit the optimum wavelength absorbed by thepolymer used and may be equipped with ventilation to avoid hightemperatures. Subsequently, the bottle is passed to an air cooling stepand is cooled to room temperature.

The coating applied to the object will generally have a thickness up to200 microns, preferably from 80 to 150 microns.

EXAMPLES

Ionomer 1:

ethylene methacrylic acid copolymer (EMAA)-Neutralised Na--10% MAA

MFI (190° C./2.16 kg)=10 dg/min.

Ionomer 2:

Copolymer EMAA-Neutralised Zn--10% MAA

MFI=30 dg/min.

Acid Copolymer 1:

Copolymer EMAA--15% MAA

MFI=60 dg/min.

Conventional oven: gas operated hot air oven, make Glasbeek, having thedimensions: 17×3×50 m with a built in conveyor belt forming 4serpentines.

Example 1

A lightweight 1 liter glass bottle (weight 415 g), maintained at 23° C.,and into which a steel rod making contact with glass is inserted, iselectrostatically sprayed with ionomer 1 powder (particle size up to 225micron).

The bottle is put, top down and rotating at 15 rpm between four ElsteinIR 2000 elements, two being locate on each side of the bottle. Thebottom of bottle is heated with a fifth Elstein IR element. The distancebetween elements and bottle is 6 cm.

The surface temperature of the elements is set at 722° C., which createsthe preferentially absorbed wavelength for this ionomer type.

The powder becomes a complete smooth melt in seven minutes. Duringcooling in ambient air no significant haze is observed. The coatingthickness is between 120 and 160 microns

Example 2

Example 1 is repeated with acid copolymer 1 powder (particle size 0-275micron). Despite a higher MFI, it requires 12 minutes to achieve goodcoating. Upon cooling, haziness is observed.

Example 3

Example 1 is repeated with ionomer 2 (particle size 75-275 micron). Nohaziness after cooling, 6 min 30 seconds until flow-out, low orangepeel.

Control--Conventional Oven

A bottle is coated according to Example 3, with the exception that aconvention oven is used in place of the IR elements. There is a heatingtime of 15 minutes to reach 210° C., flow out takes 10 minutes and afurther 10 minutes is required for cooling. The coating thickness isbetween 200 and 260 microns.

What is claimed is:
 1. A process for forming a coating of athermoplastic polymer on a hollow object formed of a low electricallyconductive material comprising applying thermoplastic polymer having amaximum IR absorption between 1300 and 1900 nm and/or 2800 and 3000 nmelectrostatically as a powder to the object while the object is held ata temperature below the melting point of the thermoplastic polymer, andthereafter heating the object by IR radiation from an IR source adjustedto the preferential absorption wavelength of the thermoplastic polymerto form the coating.
 2. A process according to claim 1 wherein thepolymer is a copolymer of ethylene and an olefinically unsaturatedorganic acid, contains from 1 to 50 weight % of unsaturatedmonocarboxylic acids and is optionally neutralized up to 80% with atleast one metal ion chosen from the 1^(st), 2^(nd) or 3^(rd) group ofthe periodic table.
 3. A process according to claim 1 wherein thepolymer is a copolymer of ethylene and 5 to 25% by weight acrylic ormethacrylic acid neutralized from 5 to 60% with an alkali metal ionselected from the group consisting of sodium, potassium or zinc.
 4. Aprocess according to claim 2 wherein the copolymer further containsbetween 1 and 40 weight % of at least one acrylate selected from thegroup consisting of methyacrylate, iso-butylacrylate, n-butylacrylateand ethylacrylate.
 5. A process according to claim 1 wherein the lowelectrically conductive material is glass.
 6. A process according toclaim 1 wherein the hollow object is at a temperature below 100° C. atthe time the thermoplastic resin is applied.
 7. A process according toclaim 1 wherein the thermoplastic polymer powder has a particle size ofup to 300 microns.
 8. A process according to claim 1 wherein the coatinghas a thickness of up to 200 microns.